1. Field of the Invention
This invention relates to silicated inorganic/organic nanoporous monolithic composites conforming to the lyotropic liquid crystalline L3 phase and the production of such composites. More specifically, this invention relates to nanoporous monolithic composites that are optically isotropic and transparent with a nonperiodic network.
2. Background of the Invention
There has been considerable focus recently on the elusive formation of mesoporous materials. This is due in part to the large gap that exists between what is able to be fabricated and what is observed in the natural world and also because of the potential uses for mesoporous materials in a wide range of applications and fields. Uses envisioned include filtration, biological separation with fine molecular weight cut off (such as for DNA separation), thin film pattern formation for use in electronics, ultracapacitors, sensors, and catalysts and catalyst supports.
A simple seashell, such as found in abalone, is based on calcium carbonate and has recently been shown to have an intricate three-dimensional mesoporous structure, which is inconsistent with the standard growth of native calcium carbonate (either rhombohedral or orthorhombic). Zaremba et al. have shown that the organic constituents control the growth of the inorganic component. By removing or adding crucial proteins the morphology of the calcium carbonate may be varied, showing the intimate connection between these components.
The shell of the abalone is an example of a biomineral; such materials are formed by a wide variety of organisms such as Echinodermata, diatoms, radiolarian, coccoliths, within the human body as well as by many plants. The extent of regulation of the inorganic material via the biological entities within the organism breaks biominerals into two classes, either biologically-induced or biologically-controlled growth, though in some cases this may be a subtle distinction. Nevertheless the requirements of both organic and inorganic components to produce such astounding materials is beyond doubt.
Biological inorganic/organic composites are most often highly porous three-dimensional structures, which may or may not be periodic. The channel system, which permeates throughout the sample, ranges in size from 1-100 nm (similar to the length scale of liquid crystalline phases formed via amphiphilic self-assembly) and is dependent upon the unique combination of the inorganic precursors and the living bio-organisms. Removal of the organic components subsequent to precipitation of the inorganic leaves the labyrinth unchanged. It would appear that this is accomplished naturally through the growth of hierarchically structured organic/inorganic composites. Soft materials (e.g., proteins, membranes and fibres) organized on appropriate length scales are used as frameworks for the growth of specifically oriented and shaped inorganic crystals with small unit cells (about 1 nm; e.g., ceramics such as CaCO3, SiO2, Fe3O4, and hydroxyapatite). The high modulus inorganic phase provides stiffness while the organic phase enhances toughness.
In addition to inorganic/organic mesoporous composites being important in the biological realm, such composites are gaining increasing significance in technological fields. Traditionally microporous materials such as zeolites (which are both naturally generated and synthetically available), in which a crystalline framework of aluminosilicates incorporate pores and cavities with dimensions ranging form xcx9c1-1.2 nm have been successfully used as catalysts, reaction environments, molecular sieves and sorption materials (due to their large internal surface areas). The global topologies of these zeolitic materials, while not directly comparable to all classes of biominerals, do contain many of the key elements: (i) three-dimensional porosity, which permeates the entire structure; (ii) monosized pores and cavities; and (iii) a strong inorganic framework providing structural integrity. However, bridging the length-scale gap between what may be synthetically produced (microporous range) and what is naturally produced in such diverse and wide splendour (mesoporous range) has been a considerable stumbling block, given the huge potential of these materials in fields ranging from electronics and quantum dot fabrication to biological implants.
A major breakthrough in this endeavor occurred in the early 1990s. For more than a century, amphiphiles have been studied for their ability to self-assembly into a plethora of different geometrical labyrinths (commonly termed liquid crystals), the length scales of which match those commonly observed in biominerals (1-100 nm). In addition some of these liquid crystalline phase structures had been compared with those of zeolites, albeit on a much larger length scale. These materials potentially have the ability therefore to act as the bridge between the micro and mesoporous inorganic materials regime. Beck et al. using a dilute solution of cationic amphiphiles (micellar solution) and inorganic precursors (aluminosilicates) successfully fabricated inorganic materials which conformed to the topological forms of the hexagonal, bicontinuous cubic and lamellar liquid crystalline phases of amphiphilic molecules, with pore dimensions of approximately 4 nm, obtained upon removal of the amphiphilic molecules. Due to the low concentrations of amphiphile in solution a co-assembly mechanism was proposed to explain the formation of the higher order topologies not normally associated with such low concentrations.
In the intervening years numerous such materials were developed. However, such materials have been severely limited by the maximum pore size able to be obtained, the necessity of removing the amphiphilic material to gain access to the pores once polymerization has occurred, the lack of predictive control over product topologies and the materials produced invariably precipitating from solution as opaque micron sized grains.
Such obstacles limit the potential of these materials. It is highly preferred that the composites be large area thin films or uniform monoliths rather than powders. By working in the concentration domains correlating to the native hexagonal, bicontinuous cubic and lamellar liquid crystalline phases in a nonionic/water system Attard et al., produced monolithic inorganic materials whose structure mirrored the original geometry of the amphiphile. The mechanism followed here being closer to temptation than coassembly, e.g., the methanol produced as a byproduct in the reaction was removed to ensure the solvent volume fraction remained constant throughout the reaction. This methodology was however restrictive in the pore dimensions able to be obtained. An additional problem exists that one must guarantee that all the byproduct (and only the byproduct) is removed during synthesis to ensure the integrity of the phase is kept.
It is an object of this invention to reproducibly produce a ceramized or silicated inorganic/organic nanoporous monolithic composite conforming to the lyotropic liquid crystalline L3 phase.
It is a further object of this invention to reproducibly produce a nanoporous monolithic composite conforming to the lyotropic liquid crystalline L3 phase that is optically isotropic and transparent with a nonperiodic network.
It is still a further object of this invention to produce an optically isotropic and transparent monolithic nanoporous inorganic of low density and refractive index with defined three-dimensional topology and fixed pore dimensions, which could be varied easily.
It is yet another object of this invention to provide a mesopore that has a continuous network of pores, that is formed from a concentrated surfactant phase, and when the inorganic precursor is added, it does not perturb the preformed structure, and the solvent, swelling agent and surfactant contained therein can be easily removed from the monolith.
It is yet another object of this invention to produce nanoporous monolithic composite materials conforming to the lyotropic liquid crystalline L3 phase that are useful in the areas of magnetism, optics, electronics and biomaterials.
It is another object of this invention to produce a nanoporous inorganic material that may be useful: in controlled filtration, in the growth of nanocomposites, as a trap for large materials (e.g., proteins), as a catalyst, as a catalyst support, as insulation, as a selective liquid barrier, as an osmotic membrane, for energy storage, as an ultracapacitor, as optoelectronic devices, for heavy metal isolation, removal, silver filtration, and/or for silver ion reduction.
Generally, the objects of this invention are achieved the production of mesoporous ceramic materials by templating the L3 phase. Broadly, a mesopore is defined as a pore size diameter in the range of 10-100 nanometers in diameter. Aerogels have nanopores, micropores and macropores. The pore surface area is comparable to that found in an aerogel, but the pore size distribution is narrower, i.e. more uniform in diameter. Broadly, the process for producing the structure comprises creating a liquid crystal, the L3 phase, coating the liquid crystal with an inorganic precursor and then converting the coated liquid crystal to a ceramic. That is what is referred to as template.
Another process for forming this mesopore materials is by coassembly. This process, for example, comprises forming an hexagonal array of surfactant microtubules, coating the microtubules with an inorganic base. The reason it is called coassembly is that instead taking a xe2x80x9ctemplatexe2x80x9d, i.e., a structure that already exists, and coating the outside with a ceramic or inorganic precursor, in coassembly you mix the inorganic precursor with the surfactant and the mesostructure then spontaneously forms.
More specifically, the objects of this invention are achieved by the ceramized or silicated inorganic/organic nanoporous monolithic composites conforming to the lyotropic liquid crystalline L3 phase and the process of producing such composites. The L pease is formed by the self-assembling of amphiphiles. The L3 phase produced has many properties, which make it particularly advantageous for use as a template in the formation of mesoporous inorganic materials. The L3 phase consists of a three-dimensional, random, nonperiodic network packing of a multiple connected continuous membrane, which evenly sub-divides the solvent into two continuous volumes. Transmission electron micrographs are consistent with this proposed random morphology. The average pore and cavity dimensions of the water domains are controlled by the solvent volume fraction, varying from 1-100 nm and permeate the entire sample. Measured characteristic dimensions are from 6 to  greater than 35 nanometers. Measured surface areas are comparable to aerogels, up to 1000 m2xc2x7gxe2x88x921. Accessible pores (which permeate the entire structure) in the silicated material correlate with the solvent domain of the original liquid crystalline phase, therefore negating the removal of surfactant in order to gain entry through these pores. This fine control leads to a very low polydispersity in these dimensions for a given solvent volume fraction. X-ray scattering studies confirm a low polydispersity in pore openings for a given solvent fraction. In addition the L3 phase is advantageously optically isotropic and water clear. The L3 phase has a viscosity comparable to that of water, which makes it easy to add inorganic precursors, which is often a problem in the more viscous, hexagonal, bicontinuous cubic and lamellar phases.
The compositions of this invention are true mesostructured silicates fomed by templating L3 liquid crystals. The pore surface areas are comparable to aerogels but with a narrow pore size distribution and a uniform, continuous pore network. The mesostructure is retained following chemical or thermal extractions.